I doubt that gold will ever drop that much in value because of that asteroid in 100 years. I think Mars is further away then Ellon Musk's romanticizing about going there leads us to believe. It's already been 60 years since we've been to the Moon and back then people were thinking "2001: A Space Odyssey" was, well, gonna be in 2001! It seems pretty obvious that expanding into space is a slower process than we imagine.

But back to the topic of gold. Let's compare gold to diamonds. There is no real reason even currently why diamonds should be so expensive, but their price is held up by the fact they are controlled by mostly just one company. Or let's look at oil. Saudi Arabia could probably sell oil for $15 a barrel and still make plenty of money, but they don't. Anyone who gets to this asteroid to mine it will have to the expense of getting there and mining it to the base cost, but would have strong interest in controlling the market (ie. the supply) and keeping the as price high as possible, such as is done with oil and diamonds.

As an aside I will add that gold will still be used in jewelry, unlike steel which rust and is hard, gold is malleable and does not rust. Diamonds on the other hand could easily be replaced by other stones in jewelry.

All this said I agree gold could fall a lot in price (just not 97%) and I doubt bitcoin is going to zero, though there is the nasty idea that someone will come along with a quantum computer that changes all the hash power equations.